In order to achieve perfect forward and perfect backward secrecy, and to counteract cryptoanalysis, the group session keys should be rekeyed.
为达到完美向前和完美向后保密以及对付密码分析，需要更新组会话密钥。
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This can happen to messages being transmitted in the clear, transmitted encrypted but stored in the clear, or decryption of messages due to stolen key or cryptoanalysis.